1 SURVEY OF THE FOULING CHARACTERISTICS BETWEEN ATTACHED AND SUSPENDED GROWTH MICROORGANISMS IN SUBMERGED MEMBRANE BIOREACTOR Po-Heng (Henry) Lee ABSTRACT The stringent discharge limits with the requirements of capacity treatment increase have necessitated for developing new processes in the field of environmental engineering. There are a lot of processes which have developed to replace conventional activated sludge system. Membrane bio-reactor (MBR) and attached process are two of the two powerful alternative processes to meet high removal efficiency with cost effective advantage. Membrane bio-reactor is a process which replaces of a secondary sedimentation/ tank by membrane units. The attached system is defined as activated sludge system that incorporates media in the suspended growth reactor. The purpose of combining these two systems in one reactor is to reduce the MBR problem and increase the attached biomass concentration to enhance the level of treatment provided. This report is to compare the attached growth and the suspended system in submerged membrane bioreactor to elevate the further applications for existing wastewater treatment plant. INTRODUCTION Throughout several decades of application and evolution of activated sludge system, this system is fairly mature and reliable. However, the activated sludge process requires a relatively low food-to-microorganism ration to ensure its stability, complete nitrification and appropriate sludge settling ability. Besides, the suspended system requires the clarifier tank to complete the treatment. Therefore, it usually employs a low volumetric loading rate and requires a large space. Plus, the slow growth bacteria, such as nitrifying bacteria, are easily to be wash out, especially at low temperature conditions and low sludge age. On the other hand, with the needs of both a population growth and stringent effluent permits, upgrading and expending existing activated sludge wastewater treatment plant is needed to be done in the near future. Cost-efficient treatment processes are to be considered recently. Hybrid systems, such as the attached system and MBR systems, are alternatives. However, the combination of the attached system and MBR process is developed to merge all of advantages in one reactor. For biological treatment of water, there are many different biofilm systems in use, such as trickling filters, rotating biological contactors (RBC), fixed media submerged biofilters, granular media biofilters, fluidized bed reactors, etc. Figure 1 shows one of the commercial media. An attached growth bioreactor was designed to minimize the effect of suspended microorganisms. Table 1 shows the existing full-scale attached system. The advantages of the attached system are (1) accumulation of high concentration of the biomass which could increase the removal rate and maintain the high sludge age, (2) the high resistance to toxic compounds and overloading. On the other hand, conventional membrane bioreactor is a system which incorporates filtration. Figure 2 shows the electron micrographs of commercial non-woven polypropylene (NWPP) and polysulphone (PS) membranes. As a result, the MBR has many advantages over conventional wastewater treatment processes. These include small footprint and reactor requirements, high effluent quality, good disinfection capability, higher volumetric loading and less sludge production. However, the MBR filtration performance inevitably decreases with filtration time. This is due to the deposition of soluble and particulate materials onto and into the membrane, attributed to the interactions between activated sludge components and the membrane. So, it has been found that the attached system has the poorer settling problem comparing to the conventional one. And MBR has a problem of fouling which clogs the membrane surface and reduce the flux. However,2the attached membrane reactor could solve the poor settling problem which is a drawback of the attached system. Also, the carriers could collide the surface of the membrane to mitigate the fouling by aeration. By combine these two systems in one reactor, the disadvantages of each system could not only keep both of advantages together but also compensate the drawbacks each other. Figure 1 Photo of (from left to right) Kaldnes type K1, K2 and K3 biofilm carriers and schematic of the moving-bed-biofilm reactor (MBBR). (Rusten et al, 2006 and Leiknes and Ødegaard 2007) Table 1 Comparison of various attached systems in wastewater treatment (Li, 2004)3 Figure 2 Electron micrographs of non-woven polypropylene (NWPP) and polysulphone (PS) membranes: (a) PS (0.3 mm); (b) NWPP (5 mm); (c) NWPP (3 mm); (d) NWPP (1.5 mm). (Chang, 2001) THE ATTACHED SYSTEM IN MEMBRANE BIOREACTOR The mechanism of membrane fouling by formation of cake layer in the membrane surface It is important to understand the formation of cake layer which causes membrane fouling in order to find a way to lessen fouling rate. As membrane filtration process starts, cake is formed on the membrane surface. The cake layer offers an additional resistance for filtration. Figure 3 and 4 shows the formation of cake layer. The permeability of the cake layer can be affected by flux, electrostatic interactions, and particle size. General observations by Petsev et al. (Le-Clech et al., 2006) include: a) When salts do not cause aggregation in the feed, the permeability of the cake layer sharply decreases with the increase in electrolyte concentration. b) The permeability of the cake layer sharply decreases with the increase in permeate flux because the increased flux results in a more compressed cake layer. c) The permeability of the cake layer increases with the surface potential of the particles due to the increase in the interparticle repulsion. However, above a certain value of surface potential, a plateau value for the permeability is reached. d) The permeability of the cake layer passes through a minimum with the increase in the particle size.4 Figure 3 Composite cake structure (Le-Clech et al., 2006) Figure 4 Fouling formation in membrane surface (Le-Clech et al., 2006) The schematics of the attached membrane bioreactor The typical attached membrane bioreactor consists of a bioreactor with a membrane module and media submerged in the bioreactor. There are blowers in the bottom of the bioreactor to supply air for the biomass and suspend the media in the bioreactor. The media are used